The present invention relates to a cooling device of an engine, in which the engine is cooled with a cooling water that is circulated in a water jacket of the engine and a radiator, a heater unit for heating a vehicle cabin and the like.
A cooling system with the cooling water that is circulated between the water jacket in a cylinder block and a cylinder head and the radiator is known as an engine cooling method. In this system, the heater unit for heating the vehicle cabin with the heated cooling water is generally disposed in a cooling-water circulation passage in parallel to the radiator.
For example, Japanese Patent Laid-Open Publication No. 2004-353632 discloses the cooling device of an engine that comprises the cooling-water circulation passage in which the cooling water is circulated between the water jacket of the engine and the radiator, the heater circulation passage that has a common passage to the cooling-water circulation passage, in which the cooling water is circulated between the water jacket and the heater unit, and the thermostat provided in the cooling-water circulation passage and operative to open and close the cooling-water circulation passage led to the radiator according to the temperature of the cooling-water fed from the water jacket. In this device, the flow passage of the cooling water led to the radiator is controlled to be closed until the cooling-water temperature increases to a specified temperature, so that the cooling water can be circulated only in the heater circulation passage. Thus, the quick warming-up of the engine is attained and the emission function of the engine is improved.
Further, the above-described device has an advantage that since the cooling water is always circulated in the heater circulation passage, the heating of the vehicle cabin can be achieved properly so as to meet the passenger's heating requirement from the engine start.
Meanwhile, since the heat exchange between the air supplied to the vehicle cabin and the cooling water is conducted at the heater unit, some heat value is taken from the cooling water at the heater unit, so that there is a limit to the quick warming-up of the engine.
Herein, in the case where the engine is restarted soon after the engine stop, the cooling-temperature and the cabin temperature would not reduce to the air temperature outside the vehicle soon, so the cooling-water temperature may be increased to a specified temperature rather promptly. In this case, if the cooling water is supplied to the heater circulation passage, the heating would be conducted at the engine start, but the engine warming-up would be delayed and therefore the properly-heated air would not be supplied to the cabin promptly. In this case, some measures to heat the engine promptly should be necessary.
Herein, in the case where the air temperature outside is relatively high, such as in summer, passing the cooling water in the heater circulation passage would bring a prolonged period of the engine warming-up, thereby deteriorating the emission function of the engine.
Meanwhile, in order to give priority to the quick warming-up of the engine, for example, there could be provided a bypass passage to bypass the radiator and the heater unit or the water pump could be stopped, so that the cooling water could remain in the water jacket and the engine could be heated promptly. However, in the case where the air temperature outside is extremely low, such as in winter, it is better that the operation of the heater unit should be available from an early stage because the cabin could be heated to a certain degree with the cooling water whose temperature has not increased sufficiently yet.
Herein, in the case where the above-described bypass passage is controlled in its opening and closing with an electromagnetic valve that is activated in response to signals from sensors, this would increase costs. Meanwhile, in the case where the water pump is stopped, some control measures to drive the water pump separately from the engine operation (without a direct connection between the pump and engine) would be necessary, this would increase manufacturing costs as well.